[llvm] [Docs][DebugInfo][RemoveDIs] Document some debug-info transition info (PR #79167)

[J. Ryan Stinnett via llvm-commits]

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+# What's all this then?
+
+We're planning on removing debug info intrinsics from LLVM, as they're slow, unwieldy and can confuse optimisation passes if they're not expecting them. Instead of having a sequence of instructions that looks like this:
+
+```text
+    %add = add i32 %foo, %bar
+    call void @llvm.dbg.value(metadata %add, ...
+    %sub = sub i32 %add, %tosub
+    call void @llvm.dbg.value(metadata %sub, ...
+    call void @a_normal_function()
+```
+
+with dbg.value intrinsics representing debug info records, it would instead be printed as:
+
+```text
+    %add = add i32 %foo, %bar
+      #dbg_value(%add, ...
+    %sub = sub i32 %add, %tosub
+      #dbg_value(%sub, ...
+    call void @a_normal_function()
+```
+
+The debug records are not instructions, do not appear in the instruction list, and won't appear in your optimisation passes unless you go digging for them deliberately.
+
+# Great, what do I need to do!
+
+Approximately nothing -- we've already instrumented all of LLVM to handle these new records ("DPValues") and behave identically to past LLVM behaviour. We plan on turning this on by default some time soon, with IR converted to the intrinsic form of debug info at terminals (textual IR, bitcode) for a short while, before then changing the textual IR and bitcode formats.
+
+There are two significant changes to be aware of. Firstly, we're adding a single bit of debug relevant data to the BasicBlock::iterator class (it's so that we can determine whether ranges intend on including debug info at the beginning of a block or not). That means when writing passes that insert LLVM-IR instructions, you need to identify positions with BasicBlock::iterator rather than just a bare Instruction *. Most of the time this means that after identifying where you intend on inserting something, you must also call getIterator on the instruction position -- however when inserting at the start of a block you _must_ use getFirstInsertionPt, getFirstNonPHIIt or begin and use that iterator to insert, rather than just fetching a pointer to the first instruction.
+
+The second matter is that if you transfer sequences of instructions from one place to another manually, i.e. repeatedly using `moveBefore` where you might have used `splice`, then you should instead use the method `moveBeforePreserving`. `moveBeforePreserving` will transfer debug info records with the instruction they're attached to. This is something that happens automatically today -- if you use `moveBefore` on every element of an instruction sequence, then debug intrinsics will be moved in the normal course of your code, but we lose this behaviour with non-instruction debug info.
+
+# Anything else?
+
+Not really, but here's an "old vs new" comparison of how to do certain things and quickstart for how this "new" debug info is structured.
+
+## Skipping debug records, ignoring debug-uses of Values, stably counting instructions...
+
+This will all happen transparently without needing to think about it!
+
+## What exactly have you replaced debug intrinsics with?
+
+We're using a dedicated C++ class called DPValue to store debug info, with a one-to-one relationship between each instance of a debug intrinsic and each DPValue object in any LLVM-IR program. This class stores exactly the same information as is stored in debugging intrinsics. It also has almost entirely the same set of methods, that behave in the same way:
+
+  https://llvm.org/docs/doxygen/classllvm_1_1DPValue.html
+
+This allows you to treat a DPValue as if it's a dbg.value intrinsic most of the time, for example in generic (auto-param) lambdas.
+
+## How do these DPValues fit into the instruction stream?
+
+Like so:
+
+```text
+                 +---------------+          +---------------+
+---------------->|  Instruction  +--------->|  Instruction  |
+                 +-------+-------+          +---------------+
+                         |
+                         |
+                         |
+                         |
+                         v
+                  +------------+
+            <-----+  DPMarker  |<----
+           /      +------------+     \
+          /                           \
+         /                             \
+        v                               ^
+ +-----------+    +-----------+   +-----------+
+ |  DPValue  +--->|  DPValue  +-->|  DPValue  |
+ +-----------+    +-----------+   +-----------+
+```
+
+Each instruction has a pointer to a DPMarker (which will become optional), that contains a list of DPValue objects. No debugging records appear in the in struction list at all. DPValues have a parent pointer to their owning DPMarker, and each DPMarker has a pointer back to it's owning instruction.
+
+Not shown are the links from DPValues to other parts of the Value/Metadata hierachy: DPValues have raw pointers to DILocalVariable, DIExpression and DILocation objects, and references to Values are stored in a DebugValueUser base class. This refers to a ValueAsMetadata object referring to Values, via the TrackingMetadata facility.
----------------
jryans wrote:

```suggestion
Not shown are the links from `DPValues` to other parts of the `Value`/`Metadata` hierarchy: `DPValue`s have raw pointers to `DILocalVariable`, `DIExpression` and `DILocation` objects, and references to `Value`s are stored in a `DebugValueUser` base class. This refers to a `ValueAsMetadata` object referring to `Value`s, via the `TrackingMetadata` facility.
```

https://github.com/llvm/llvm-project/pull/79167